The present invention relates to a method of cleaning a plurality of cooling pipes in a heat transfer unit in a steam power generating plant and, more particularly, to a method of cleaning the cooling pipes by circulating a number of cleaning bodies, such as sponge balls, through the cooling tubes.
The invention is also concerned with a cleaning system suitable for carrying out the cleaning method.
Generally, a steam power generating plant has a condenser incorporating a plurality of cooling tubes which open at their one ends to a cooling water inlet chamber and at their other ends to a cooling water outlet chamber defined in the condenser. A cooling water inlet pipe and a cooling water outlet pipe are connected to the cooling water inlet chamber and the cooling water outlet chamber of the condenser, respectively. The cooling water is supplied by a cooling water supply pump into the cooling water inlet chamber of the condenser through the cooling water inlet pipe, and is distributed over all cooling tubes to flow therethrough to reach the cooling water outlet chamber from which it is discharged through the cooling water outlet pipe.
In a steam power generating plant of ordinary electric power station or nuclear power station, sea water is usually used as the cooling water for the condenser. The sea water generally contains various foreign matters such as slime, marine animals and so forth. Consequently, these foreign matters attach to the inner surfaces of the cooling tubes to contaminate the latter, resulting in a lowered heat transfer across the walls of the cooling tubes. As a result, the heat exchanging performance of the condenser is deteriorated to lower the level of the vacuum established at the steam side of the condenser, which, in turn, undesirably elevates the back pressure of the power generating tubrine to lower the power generating efficiency of the plant as a whole. To avoid this, the cleaning of the inner surfaces of cooling tubes is essential.
The cleaning of the cooling tubes is achieved by circulating a number of cleaning bodies, such as sponge balls, through the cooling tubes together with cooling water.
In the conventional cooling tube cleaning system, the cleaning bodies are charged into the cooling water inlet pipe of the condenser and moved to the cooling water inlet chamber from which they pass through the cooling tubes to reach the cooling water outlet chamber and are then discharged from the condenser through the cooling water outlet pipe. The cleaning bodies are finally collected in the cooling water discharge pipe.
There has been no attempt made to quantitatively grasp the contamination of the cooling tubes in the condenser. Namely, in the conventional method, various data such as level of vacuum in the condenser, inlet temperature of the cooling water, outlet temperature of the cooling water, delivery pressure of the cooling water supply pump and so forth are observed independently. The cleaning is carried out when there is any sign of cooling tube contamination, such as lowering of the condenser vacuum, reduction of the temperature difference of cooling water at the inlet and outlet sides, rise of the pump delivery pressure and so on. When the sea water is used as the cooling water, the amount and type of the contaminants such as slime and marine animals differ depending on the season. Some of the contaminants may accumulate drastically in the cooling tubes to cause a rapid contamination.
It is, therefore, impossible to achieve an effective cleaning of cooling pipes, with the conventional system in which the contamination of the cooling tubes is qualitatively judged through observation of independent data. Consequently, the efficiency of the power generating plant has been undesirably lowered due to an inadequate management of the cleaning of the cooling tubes.
For maintaining the condenser in clean state as much as possible, it has been a common measure to conduct the cleaning in accordance with an annual cleaning schedule or plan which is worked out beforehand taking into consideration the seasonal change in the amount and types of contaminants brought into the condenser. In other words, the frequency or demand for the cleaning work varies according to the season. This qualitative determination of the contamination, i.e. the qualitative mangagement of the cleanliness, cannot provide effective and satisfactory cleaning of the cooling tubes. In order to maintain a constant performance of the steam power generating plant, it is highly desirable to exactly ascertain the cleanliness of the cooling tubes and to carry out the cleaning whenever it becomes necessary, i.e. when the cleanliness goes below a predetermined limit of allowance.
U.S. patent application Ser. No. 213,095, filed on Dec. 4, 1980, now U.S. Pat. No. 4,390,058, proposes an improved tube cleaning method wherein the heat flux across the tube wall is measured by heat flux sensors attached to some of the cooling tubes, while the temperature difference of cooling water between the inlet and outlet sides of the condenser is measured by means of temperature sensors provided in the cooling water inlet pipe and the cooling water outlet pipe, respectively. Then, the total heat transfer coefficient is calculated from the measured data to provide an index of the actual cleanliness of the cooling tubes, to inform the operator of the change or timing for the cleaning of the tubes. This method satisfies to some extent the demand for adequate cleaning of the cooling tubes. This method, however, is still unsatisfactory, although it permits a qualitative determining of the tube cleanliness and selection of timing for the effective cleaning, due to the reason stated below.
Namely, all of the plurality of cooling tubes in a condenser do not always have equal degree of contamination. The cleaning bodies such as sponge balls are once introduced into the cooling water inlet chamber and then distributed to the plurality of cooling tubes. This means that some of the cooling tubes can receive sufficient number of cooling bodies while the others can not, mainly due to the influence of state of flow of water. Thus, it is impossible to clean all cooling tubes equally, with the conventional cleaning method which treates all cooling tubes as a group.
For cleaning all cooling tubes equally and satisfactorily, it is necessary to determine the state of contamination locally and quantitatively and to effect the cleaning with a suitable weight of cleaning power on each local point of contamination.